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I am trying to get my car buttoned up before going for a tune next weekend. I am having trouble with the speedometer and fuel gauge not work, both of which I don't believe have to go through the ECU. My question is if they don't need ECU assistance, why would they not work? The only thing different from when the stock engine was in is the lack of OEM ECU. The only thing I can think of is maybe one of those wires were grounded on the factory ECU and now they are not. The fuel sender has the 2 wires Blue and Brown/White and it has continuity right through to the cluster. The VSS is a 3 wire Power, Ground, Signal and same thing, it has power and I ground it straight to the engine bay but no input on the cluster. Can anyone point me in the right direction?

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The fuel senders on most subaru's take 5v from the ECU, but the output from the sender to the gauge. Think of the senders like a pull-down to ground based on variable resistance. There are also 2 fuel sender modules connected in series and if the second one is disconnected, you will have no ground connection. join your 5v regulated output onto the BrB (BrW in your case) wire between the gauge and the sender and it should work.

The speed sensor probably does the same thing and expects a 5v pullup somewhere in the circuit. Try wiring this into a DI on the ECU and setting pullup to ON. will fix your speedo + give you vehicle speed into the ECU. In the below diagram, C goes to ground.

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thanks for for the info. It seems as though giving the BrW +5v has some effect on the fuel pump because the car will not run with RedL wire hooked up. I’m not sure about the speedometer as I haven’t driven it yet. Any idea why the fuel pump would stop working?

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It is a 2000 GM6. 2 door USDM Normally aspirated 2.5 SOHC. There is the fuel pump on one side with a 6 wire plug that jumps over to a 2 wire plug on the other side of the fuel tank. None of that stuff was changed with the swap. I have only tapped into chassis wires that I needed.

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Ah ok, the last one I had to fight with the fuel system wiring on was a legacy but they are usually pretty similar.

Here are the relevant pages from the service manual relating to fuel gauge

First thing i'd point out is that with you only finding 2 wires on the module you're looking at the sub module.

so working backwards through that second diagram, BlackYellow is ground. As long as you can confirm ground on this wire where it meets the primary level sender this is not complicated. RedYellow is the low fuel level light, I believe this goes to ground when the level is low, but it doesnt participate in the level gauge anyway so not relevant to this issue. This leaves BrownWhite as the only wire leaving the sender pair. Blue is simply a join between the 2 and simply needs to be checked for continuity.

Thinking about this, the way to verify whether the 5v pullup is coming from the ECU or from that "custom CPU" in the gauge cluster, is to:

1) verify the wiring to ground and between the 2 fuel senders. with the ECU and gauge disconnected you should be able to measure a resistance between the BrW wire where it would plug into and ground, and this resistance should change if you push the level sensors up and down. This confirms your wiring through the level sensors.

2) Disconnect all the plugs from the gauge cluster Verify that the relevant pins on the plugs show 12v and ground to confirm that the dash is "powered up"

3) disconnect the ECU from that circuit but connect the dash. Now with the key off you should see exaclty the same behaviour as above. With the key on, if you see the same 0V and varying resistance to Ground, then the ECU needs to provide the 5v pullup. If you see 0-5V ish as per the ECU function for pin B136-27 above, then the gauge is providing 5v pullup internally and your problem is either in the dash or on the main power/ground feeds into the dash.

I also noted while checking these over that the fuel level senders and the speed sensor share some of the same wiring on the power/ground feeds, so maybe your problem is as simple as a fuse or ground wire not connected? quick check with a multimeter with key on will tell you if you see 12v bewteen the 2 non-signal pins of the speed sensor.